The phon was proposed as a unit of perceivedloudnesslevelLN for pure tones[1] by S. S. Stevens. The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones.[2] By definition, 1 phon is equal to 1 dBSPL at a frequency of 1 kHz.[3] The equal-loudness contours are a way of mapping the dBSPL of a pure tone to the perceived loudness level in phons. These are now defined in the international standard ISO 226:2003, and it should be noted that the research on which this document is based concluded that earlier Fletcher–Munson curves and Robinson-Dadson curves were in error.

The phon model can be extended with a time-varying transient model which accounts for "turn-on" (initial transient) and long-term, listener fatigue effects. This time-varying behavior is the result of psychological and physiological audio processing. The equal-loudness contours on which the phon is based apply only to the perception of pure steady tones; tests using octave or third-octave bands of noise reveal a different set of curves, owing to the way in which the critical bands of our hearing integrate power over varying bandwidths and our brain sums the various critical bands.